Traditional windmills comprise propellers that rotate in the vertical sense in parallel to the equipment sustaining post, and are directly connected to the generator shaft or comprise shells that rotate in the horizontal sense perpendicularly to the post, connected to a vertical shaft.
In the case of propellers, they rotate as a function of a component (fraction) of the force of the wind that provides a rotation in the sense perpendicular to the wind direction. The propellers' aerodynamic surfaces take advantage of only a component of the wind force, and they possess extremely small area if compared to the circle described by their movement.
Propellers were developed basically to produce displacement of a mass of air, in other words to move the wind, and not to be moved by the wind, and so they are of low efficiency.
It being known that the wind acts to produce a force proportional to an area; if the area of wind reception is larger the “lifting” force of the wind will be larger. According to the conception of propellers, they possess little aerodynamic surface for wind reception, which is worsened by the impossibility in using several propellers in the same axis, due to aerodynamic interactions and the great structural limitation that the 7 propellers are singly supported on the spinner. This support will be supporting the weight of the propeller and also the wind load reaction. For example, a propeller with only 37 meters (circle radius) and weighing 7 tons approximately and also loaded with 1.47 tons from the wind, will produce a reaction at the junction of the propeller with the spinner of 8.47 tons, which is a limiting factor for the expansion of the wind reception area and it is decisive for the high cost of production of this system that is reflected directly in the calculations of competitiveness of wind energy.
In embodiments comprising shells, the problem is more serious, because they possess the same aerodynamic surface on their front part and their back, only with different resistance coefficients, resulting in an extremely small rotational force that reduces even more the efficiency of this method of wind force reception.
Another technical problem with the converters of the state of the art is the fact that the generated rotation is directly proportional with the speed of the incident wind, without the possibility of effective control of the rotation of the propeller shaft.
So, the power generated by generators moved by these converters oscillates, which is not desirable for generation of energy for consumption on a large scale.
The state of the art also includes converters endowed with blades or panels fixed to a horizontal shaft that rotate on a vertical axis, as in the document DE8701224. These panels are fixed to the horizontal shaft so that the panel is divided in two parts, a part above the shaft and another below, with different dimensions. When the wind beats on the front of the panel, the panel is in the vertical position, rotating the main shaft. When the panel arrives in the position that the wind beats in its back, the panel rotates to be in the horizontal position in a way that it does not have resistance to the wind (“feathering”). However, this mechanism is ineffective, because when the panel is with its back to the wind to be moved to the horizontal position, the part of the panel that is above of the horizontal shaft resists the feathering movement by the force of the wind, it being necessary to have an equal panel area below the horizontal shaft for the resultant force to be still zero kilograms and another part of the area will be used to combat the weight and drag in a way to permit the feathering movement. That is, in this embodiment, all the area that uses the wind pressure that beats on the inactive panel to feather it corresponds to an equal area lost to push the traction panel on the active side. A similar situation occurs to move the panel out of the horizontal position, which will reduce the return time to the vertical position, creating a new loss in relation to the total angular use of the traction side (180°) of the wind force.
As the panels are mechanically linked by the same shaft, arranged at 90° degrees and the wind is not always even due to bursts or turbulences, this link between the panels will increase even more the lifting wind force lost because the effort to move one panel can be added to an asymmetrical wind load from the opposite side by the other panel.
Such presented problems reduce considerably the conversion of the wind force into rotational force on the main vertical shaft, with the result that transformation into rotational energy is very small if one compares the total area for lift with the areas that offer resistance to the feathering and unfeathering, and this system performs inferiorly to propellers. One estimates that only 30% of the lift obtained by the traction panel is used. Furthermore, this embodiment has the same limitations on expansion of the wind reception area for generation of energy on large scale, due to the fact of the panels horizontal shaft is singly supported in the vertical axis, with the attending structural problems, functional problems and high costs for the size (length) of the panels to reach significant dimensions, as in the propeller embodiments.
The solutions of this state of the art try to compensate such problem making the panels with materials of different weight above the horizontal shaft and below the horizontal shaft, so that the upper part of the panel is heavier than the lower part. However, that is irrelevant in relation to the aerodynamic factors where the wind opposes the panel movement.
Also, this system does not foresee rotation control, it is not equipped with means for the movement of a panel toward the main shaft or away from it, it does not foresee a system for varying the panel area to compensate for wind speed variation, it does not foresee a brake to stop the system for maintenance, it does not allow feathering the panels in case of a windstorm, it does not present the support structural part of the rotary mast linked to the panels, and this embodiment is limited by having only two planes of panels interlinked with each other at a 90 degree angle.
In relation to the system presented in the document WO2003/014565 one finds practically the same limitations of the document DE8701224, the difference in these embodiments just being the relationship to the positioning of the horizontal shaft.
However, the embodiment of WO2003/014565 also has its horizontal shaft singly supported and so does not allow the use of this system for the generation of energy on large scale, and this embodiment starts to have a big loss in the useful lift due the effort for feathering in function of the weight; not so only by the wind action, but also in function of the structural shape of the panel, because such panel in accordance with the drawing will resist the wind aerodynamic pressures even if constructed using light materials such as aluminum, carbon fiber, etc., it will possess significant weight per square meter of surface compared to the wind lift produced per square meter.
So, the system of WO2003/014565 as described does not counterbalance the necessary wind pressure for the feathering and will reduce significantly the lift resulting, mainly due to resistance to the wind flow.
As the panels are mechanically interlinked on the same shaft by 90 degrees, and the wind is not usually even due to bursts or turbulences, this interdependence among the panels will increase still more the lift losses because the effort to move a panel can be added to an asymmetric wind load on the side opposed by the other panel, which is worsened as shown in the drawings and the description, by the angle of 90° formed (between the lifting traction panel (active side) and the inactive side) through the interdependence shaft that rotates together with the panels, in such a way that the horizontal panel is totally flat, and without any decline angle (that would provide exit from horizontal position and free from aerodynamic cost) that results in the formation of a mattress of air that produces great air resistance for that panel to leave from the horizontal position to the vertical position of traction.
Such problems reduce the use of the wind force considerably for the transformation into rotational force to the main vertical shaft, whose resultant force is very small if one compares the load for lifting with the weight of the areas that offer resistance for the horizontal and vertical positioning of the panels, added to the aerodynamic drag mentioned. So, this system possesses inferior performance compared with propellers, one estimates only 30% of the traction panel lift is used due to loss with the horizontal and vertical positioning of the panels. Furthermore, this embodiment has the same limitations as propellers or panels in the structural and functional point of view for energy generation on large scale, due to the fact of the horizontal shaft of these panels is singly supported to the vertical shaft through bearings, resulting in limitation for the size (length) of the panels to reach significant dimensions. Furthermore, the production of a singly supported panel with this configuration would have high production cost similarly as for a propeller.
This system does not foresee rotation control, it is not equipped with means for movement of the panels toward or away from the vertical shaft, it does not foresee a system of variation of the area of the panels to compensate for wind speed variation, it does not foresee any brake to stop the system for maintenance, it does not allow any means to put the panels in a horizontal position in case of a windstorm, it does not present the structural part of a rotating mast support linked to the panels, and this system is limited to only two planes of panels interlinked at 90 degrees.
EP0379626A1 describes an embodiment having a form of reception of the force of the winds that presents several limitations to good performance between the lift produced on the side that the wind pushes (producing traction) and the inactive side.
The system is composed of panels formed by several interdependent foils, arranged like slats of venetian blinds, that move through 90° between the active side (in front of the wind to produce lift) and the inactive side (back).
The foils of a same quadrant move simultaneously because they are interlinked through a belt. This belt transmits the rotating movement to put the slats in a horizontal or vertical position that arranges the slats through a mechanical command, that, as shown in the drawings, uses the rotation of the system (through gears, bearings, arms, etc) to make simultaneously (on the active and inactive side) the movement to the horizontal and vertical positions of the slats that compose each panel. The foils move through a complete course each ½ rotating cycle of 180°, in way to mechanically determine the horizontal or vertical position of the foils in a mode synchronized with the rotation of the group.
In the embodiment of EP0379626A1, the feathering movement is not determined directly by the direction or pressure of the wind, because the wind that acts on the aerodynamic panels does not have the vertical positioning function or horizontal positioning function in the system.
To compensate such problem in the way of the system to put the foils in the vertical position and to put the foils in the horizontal position in function of the direction of the wind (because otherwise it would not work) the illustrations show a small rudder that when being rotated by the wind modifies the mechanical parameters to try to synchronize the direction of the wind with the correct moment for the movement of the slats between the vertical and horizontal positions.
The rudder presented in the figures will only be capable to redirect the mechanical command for the movement of the foils in a way to compensate changes in the direction of the wind in extremely small systems. In other words, this system would not work for power generation on a large scale. Observe that a system with propellers seventy meters in diameter provides an approximate load of 23 tons owing to the weight of the propellers and wind loading, and to reposition such load when the wind changes direction, a powerful electric or hydraulic motor, and never a rudder as shown in EP0379626A1, is needed to change the instant to put the foils in the horizontal and vertical positions in relation to the change of the direction of the wind.
The system of the embodiment of EP0379626A1 also presents the same deficiencies already described in the documents DE8701224 and WO2003/014565 regarding the interdependence among the slats with an angle of 90°, (without declination angle) and aerodynamic costs, the weight of the foils that do not possess counter-balances, and all of these worsened by the attrition and weight of the mechanical system for feathering, and by any inefficiency caused by misalignment of the wind direction and the timing of the mechanically commanded feathering. In addition this system does not foresee rotation control, it is not equipped with means for movement of the panels toward or away from the main vertical shaft, it does not foresee a system to vary the area of the panels to compensate for alteration of the wind speed, it does not foresee any brake to stop the system for maintenance, it does not allow feathering in case of a windstorm, and it can be estimated that, in comparison with a shells system, it acts with a benefit of around 18% in the traction force produced by the wind for transformation into rotational mechanical energy.
The solutions of this state of the art try to compensate such problem making the foils with materials of different weight, so that the top part of the foil is lighter than the lower part. However, as a result, when the panel has to be in the horizontal position, the lower part being heavier demands a wind with larger force to maintain it in the horizontal position and even altering the weight of the lower and upper parts of the foils the problem becomes worse. Therefore this hinders still more the feathering of the foils, since what determines the horizontal position are the resultants of the wind against and in favor of the horizontal positioning movement.
Another technical problem of this system for generation of energy on a wide scale arises from the lack of a control that moves the panels toward or away from the main rotation axis to effectively control the rotation to compensate for variation in the wind speed.